The Effect of Aza-Substitution on the Photoprotective Properties of Pyrimidine Nucleobases

The canonical nucleobases, which form the building blocks of our genetic coding material, are known to protect themselves against photodamage through ultrafast internal conversion processes that dissipate potentially harmful UV energy into heat. However, seemingly minor single atom substitutions can profoundly alter the photophysics of the nucleobases. Most notably, inaccessibility of conical intersections between the singlet excited states and the ground state can lead to highly efficient intersystem crossing into the triplet manifold.

Azabases, where a ring carbon has been replaced by nitrogen, have been classified in two groups based on their tendencies for intersystem crossing. Type 1 show a high triplet quantum yield, whereas type 2 internally convert to the ground state. We use time-resolved photoelectron spectroscopy to investigate the electronic relaxation mechanisms in 6-azauracil and 5-azacytosine, previously categorized as type 1 and 2, respectively. For the latter, we also distinguish tautomer effects to refine this classification.